The present invention relates to a capacitor having a capacitor insulating film composed of a ferroelectric film or a high dielectric constant film and in particular a capacitor used for a ferroelectric memory or a high dielectric memory, and a method for fabricating the same.
As for ferroelectric memories, mass-production has started for ferroelectric memories of a planer structure having a capacity of 1 to 64 Kbits. In recent years, ferroelectric memories of a stack structure having a large capacity of 256 Kbits to 4 Mbits have become dominant in development. To implement stack-type ferroelectric memories, it is indispensable to substantially improve the scale of integration and hence provide finer ferroelectric memories. To achieve this, it is important to secure conformity among the step of forming ferroelectric capacitors, the step of forming transistors and the wiring step.
In relation to the above, the following problem arises. That is, ferroelectric capacitors must keep their ferroelectric films from being reduced to maintain the polarization property of the ferroelectric films in a semiconductor process, in which processing is often performed in a hydrogen atmosphere, as is typified by processing of embedding contacts using W-CVD and heat treatment in a hydrogen atmosphere for recovery of the properties of transistors, for example.
FIG. 18 shows the degree of degradation of the polarization observed when heat treatment at a temperature of 400° C. in a hydrogen atmosphere, which is normally adopted in the wiring step for semiconductor devices, is performed for a ferroelectric capacitor having a bottom electrode made of Pt, a capacitor insulating film made of SBT and a top electrode made of Pt. In FIG. 18, the y-axis represents the polarization 2Pr (μC/cm2) and the x-axis represents the hydrogen mixture ratio.
As is apparent from FIG. 18, degradation of the polarization begins when the hydrogen mixture ratio is 0.001%, and a complete short circuit occurs when it is 0.5% or more. From these results, it is found how a ferroelectric film is sensitive to hydrogen.
Conventionally, a technique of coating a ferroelectric capacitor with a hydrogen barrier film is generally adopted, as is described in Japanese Laid-Open Patent Publication No. 11-8355, for example. By this coating, diffusion of hydrogen generated in a semiconductor process after formation of a ferroelectric capacitor is blocked with a hydrogen barrier film, which is typically an Al2O3 film, and thus degradation of the polarization of the ferroelectric film is prevented. This effect is provided most greatly for a structure of the ferroelectric capacitor completely covered with the hydrogen barrier film. In this way, degradation of the polarization property of the ferroelectric capacitor due to hydrogen is prevented, and thus a highly integrated ferroelectric memory or high dielectric memory is realized.
A conventional ferroelectric capacitor of a completely covered structure will be described with reference to FIG. 19.
Referring to FIG. 19, a first interlayer insulating film 201 is formed on a semiconductor substrate 200 on which a memory cell transistor (of which illustration is omitted) is formed, and a first hydrogen barrier film 202 is formed on the first interlayer insulating film 201. On the first hydrogen barrier film 202, formed is a ferroelectric capacitor having a bottom electrode 203, a capacitor insulating film 204 composed of a ferroelectric film, and a top electrode 205. The top surface and side faces of the ferroelectric capacitor are covered with a second hydrogen barrier film 206. Thus, the ferroelectric capacitor is completely covered with the first hydrogen barrier film 202 and the second hydrogen barrier film 206.
A second interlayer insulating film 207 is formed covering the second hydrogen barrier film 206, and a metal interconnection 209 is formed on the second interlayer insulating film 207 via a barrier layer 208. A first contact plug 211 extends through the first interlayer insulating film 201 and the first hydrogen barrier film 202, for connecting the memory cell transistor with the bottom electrode 203. A second contact plug 212 extends through the second interlayer insulating film 207 and the second hydrogen barrier film 206, for connecting the top electrode 205 with the metal interconnection 209.
Since the conventional ferroelectric capacitor is completely covered with the first and second hydrogen barrier films 202 and 206, diffusion of hydrogen into the capacitor insulating film 204 is suppressed even when the ferroelectric capacitor is subjected to heat treatment in a reducing atmosphere. In this way, degradation of the polarization property of the ferroelectric film constituting the capacitor insulating film 204 can be reduced.
In the prior art described above, the ferroelectric capacitor was completely covered with the hydrogen barrier films. For reducing the number of steps, reducing the cost and facilitating the integration, there is also known a structure of the ferroelectric capacitor of which only the top surface is covered with a hydrogen barrier film.
The conventional ferroelectric capacitor described above however has the following problem. The inventors of the present invention performed heat treatment for a ferroelectric capacitor covered with hydrogen barrier films as described above in a reducing atmosphere, and found that it was not possible to prevent completely the degradation of the polarization property of a ferroelectric film constituting the capacitor insulating film.
FIG. 20 shows the results of the properties of a ferroelectric capacitor covered with hydrogen barrier films, measured before and after heat treatment performed for the ferroelectric capacitor in a hydrogen atmosphere. In this experiment, heat treatment in a 4% hydrogen atmosphere was performed for a ferroelectric capacitor having the structure shown in FIG. 19, in which a Pt/IrOx/Ir/TiAlN laminated film was used as the bottom electrode 13, an SBTN film was used as the capacitor insulating film 14, a Pt film was used as the top electrode 15, a plasma silicon nitride film was used as the first hydrogen barrier film 12, and an aluminum titanium oxide film was used as the second hydrogen barrier film 16. In FIG. 20, 2Pr represents the polarization (μC/cm2), Pnv represents the saturated polarization (μC/cm2) observed when the no-voltage applied state continues sufficiently long, and BVF represents the breakdown voltage (V) observed when a current of 10−7 A/cm2 flows to the ferroelectric capacitor.
Considering the experiment results shown in FIG. 18, it is found that in the experiment shown in FIG. 20, 0.001% or less of hydrogen entered the ferroelectric film and the concentration of hydrogen entering the ferroelectric film was reduced to 4000/1 or less thanks to the hydrogen barrier films. From the experiment results shown in FIG. 20, it is also found that degradation of the ferroelectric film cannot be completely prevented even when the ferroelectric capacitor is covered with the hydrogen barrier films.
In view of the above, the present inventors have examined paths through which hydrogen enters the ferroelectric film, and found that hydrogen diffuses into the ferroelectric film via a first diffusion path passing via the first contact plug 211 and the second contact plug 212, a second diffusion path composed of an interface between the first hydrogen barrier film 202 and the second hydrogen barrier film 206, and a third diffusion path composed of grain boundaries of the first hydrogen barrier film 202 poor in crystallinity due to a step formed on a layer underlying the first hydrogen barrier film 202.
As for the first diffusion path via the contact plugs, conductive hydrogen barrier films may be provided right above the top electrode and right below the bottom electrode. However, for the remaining diffusion paths, there exist problems considerably difficult to solve that the crystallinity must be improved at the step and that the cohesion between the dense hydrogen barrier films must be improved.
As described above, even when a ferroelectric capacitor is completely covered with hydrogen barrier films, it is not possible to prevent completely degradation of the polarization property of a ferroelectric film constituting the capacitor insulating film of the ferroelectric capacitor when the ferroelectric capacitor is subjected to heat treatment in a reducing atmosphere.